Solid-state imaging readers have been installed in slot scanners or workstations having a horizontally and/or a generally vertically arranged window in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years, to electro-optically read symbol targets, such as one-dimensional and two-dimensional symbols, particularly a Universal Product Code (UPC) bar code symbol, and non-symbol targets, such as driver's licenses, receipts, signatures, etc., the targets being associated with products to be processed by the slot scanners. An operator or a customer may slide or swipe a product associated with a target in a moving direction across and past a window of the slot scanner in a “swipe” mode. Alternatively, the operator or the customer may momentarily present the target associated with the product to an approximate central region of the window in a “presentation” mode. The choice depends on user preference or on the layout of a workstation in which the system is used.
The reader includes an imager having a one- or two-dimensional array of sensors (also known as pixels), which correspond to image elements or pixels in a field of view (FOV) of the array. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and is analogous to the imager used in a digital consumer camera to capture images. The reader includes an illumination system for illuminating the target with illumination light emitted from a light source and directed through the window to the target for reflection and scattering therefrom, as well as an imaging lens assembly having an optical axis for capturing return light from the target through the window, and for projecting the captured return light as an image onto the array. Electronic circuitry, including a controller or programmed microprocessor, processes electrical signals generated by the array and indicative of the captured image. If the target is a symbol, then the controller also decodes the captured image and thus reads the symbol. If the target is not a symbol, then the controller processes, for example, stores, optically corrects, and/or displays, the captured image.
The position and orientation of the target relative to the window are variable. The target may be located low or high, or right to left, on the product, or anywhere in between, or on any of the sides of a box-shaped product. The target may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between. The target may be movable or held momentarily steady relative to the window. The target may be located on, near, or remote from, the optical axis of the imaging lens assembly, and may be located anywhere either inside or outside of the FOV of the array, and may be located anywhere inside or outside a range of working distances along the optical axis away from the window.
It is desirable to have the FOV of the array relatively large at a near working distance or a close proximity to the window of the workstation so that the FOV covers the entire target. At farther working distances, it is preferred to have the FOV diverge slowly. As advantageous as the use of the solid-state reader is, the range of working distances or ability to read targets by the reader is limited by the optical characteristics of the imaging lens assembly and by the number of pixels in, or resolution of, the imager array. A ratio between the smallest imaged bar/space area, or module, of a target bar code symbol to the number of pixels on which the bar/space area is projected onto the imager array, that is, the number of pixels covered by the bar/space area, is known as “pixels per module” or PPM. When the FOV diverges rapidly, the PPM decreases rapidly too, which, in some cases, limits the capability of the reader to decode and read target symbols at far working distances.
In cases when the target symbol is tilted in a particular direction at a substantial imaging angle, for example, 45 degrees or more, relative to the optical axis of the imaging lens assembly described above, and when the target symbol is located at outer peripheral edge regions of the FOV radially remote from the optical axis, the PPM is further reduced as a function of the cosine of the imaging angle due to the projection effect. Also, the tilted symbol appears to look denser than it actually is. A projected image of the tilted symbol on the array has a higher density than the image of a symbol oriented in a plane generally perpendicular to the optical axis of the imaging lens assembly. It is desirable to increase the PPM along the particular direction of the tilted target symbol to compensate for the loss of the PPM, the reduced resolution, the apparent increased density, and the decreased capability to decode and read such tilted symbols located in such radially remote outer peripheral edge regions of the FOV.